Causes of water pollution are varied:

Industrial effluents; have heavy metals that poison aquatic organisms;

Untreated organic matter has phosphates/sulphates/nitrates/salts; that cause eutrophication; causing algal bloom that deprives the water of nutrients; when the algae die,
they lead to an increase in putrefying bacteria whose decomposition activities lead to the release of awful smells/odours;

Oil effluents clog respiratory surfaces of aquatic organisms/death due to suffocation;

Domestic effluents/sewage; form a habitat of pathogens that spread water borne diseases; decomposing sewage promotes eutrophication leading to
algal bloom; death promotes/attracts saprophytic bacteria that use up oxygen in water; causing suffocation/death to aquatic organisms;

Agrochemicals/fertilizers; phosphates/nitrates; cause eutrophication; heavy metals in agrochemicals (herbicides/pesticides); affect respiratory surfaces/cause breathing problems;

Hot water; raise temperature of water; killing organisms; dissolves less oxygen; reducing its content in water;

Oil spillage; in oceans from tanks/refineries; soak feathers of marine birds preventing flight; clogs respiratory surfaces leading to death; coats photosynthetic phytoplanktons; reduces light penetration hampering photosynthesis;

Sediments; from soil erosion makes water dirty; making it unfit for consumption; clogs respiratory surfaces hindering gaseous exchange; reduces light penetration hindering photosynthesis;